1. Field of the Invention
In its general aspect, the present invention relates to a process for the preparation of a composite polymeric material containing uniformly dispersed, nanometric inorganic inclusions.
In particular, the invention relates to a process of the aforesaid type, wherein said inclusions are obtained “in situ”, i.e., they are produced in the presence of the polymer.
The invention also relates to a formed article realized in the above composite polymeric material.
Hereafter in the description and in the following claims, the words “formed article” mean any manufacture or finished plastic product such as sheets, bars, tubes, extruded objects, molded parts and similar products obtained through transformation of granules, tablets, spheroids or powders of thermoplastic polymers or of specific thermosetting resins, as it will be more apparent hereafter in the description.
2. Description of the Related Art
As it is known in various fields of the technique, the interest in high technology materials is greater and greater, such as for example composite materials able to ensure high performances.
The technological importance of a composite material lies in that it has properties that are superior, or somewhat characteristic, compared to the properties of the single components it is made of.
Among composite materials, the so-called nanocomposites, i.e., materials comprising at least two components where one is dispersed in the other in the form of nanometric particles, represent the most recent and interesting development.
Such materials enjoy, in fact, peculiar characteristics conferred by the material reduced to nanometric dimensions, and they exhibit therefore several fields of use.
The peculiar characteristics are due to a substantial change of the group of chemical-physical properties the material undergoes when it is reduced to the nanometric dimension.
The deep change is mainly due to the neat prevalence of the characteristics of the surface atoms on those of the inner atoms. Metals stay among the elements which mainly show an alteration of the properties linked to the dimension.
For example, a change has been shown in the magnetic behavior of a material in the passage to the nanometric dimension. In particular, by reducing the dimension of a ferromagnetic material, there occurs first an increase of the coercive field Hc and subsequently a superparamagnetic behavior (Hc=0). This variation of the magnetic properties finds application, for example, in the realization of mass memories, magnetic refrigerators, magnetic sensors and transformers.
Also the optic properties of the material are strongly influenced by the nanometric dimension. Some metals such as gold, silver, copper exhibit, when reduced to the nanometric dimensions, a typical phenomenon known as plasmon effect which consists in an oscillation of the surface electrons under the action of an incident wave. This effect results in an absorption at a predetermined wavelength in the visible spectrum, which allows the use of these materials as optic filters, polarizers, optic sensors, etc.
Nanocomposite polymeric materials are of great interest, i.e., materials wherein nanometric inorganic inclusions are dispersed in a polymeric matrix. The polymers can, in fact, stabilize the nanometric phase and thus the above properties without altering them in a significant way, moreover they can improve their own performance as hereafter described.
A nanometric inorganic phase inside the polymeric matrix can increase the thermal exchange capacities or the electric susceptibility of the polymer, improve the barrier properties against gas, or even produce shielding properties against radiations, antistatic properties, having effects on the ignition temperature of the polymer. Also the mechanical and optic properties of the polymer can be changed and improved by the presence of nanometric inclusions.
From the preparation point of view, nanocomposite polymeric materials, in particular composites comprising metallic inclusions, can be obtained by means of two different processes defined “ex-situ” or “in-situ”, depending on the nanoparticles being first produced and then incorporated into the polymer or obtained directly inside the polymeric matrix by using a precursor.
The processes “in-situ” in turn differentiate depending on the precursor being transformed in the desired inclusions by using chemical agents, for example with hydrogen, hydrazine, boron hydride, or through the physical means of thermolysis, photolysis or radiolysis.
A process for the preparation “ex-situ” of noble metal nanoparticles is described in U.S. Patent Application No. 2002/0034675 by Starz et al. In such document nanoparticles are described containing noble metals, alone or in combination with common metals, passivated by immersion in an aqueous solution of a polysaccharide serving as temporary stabilizer. Particles thus obtained are then used for the realization of electrodes for polymeric membrane fuel cells or as catalysts for other types of applications.
Although advantageous, the processes “ex-situ” are rather complex and limited in that the nanometric material is extremely reactive and therefore difficult to be handled. The inclusions must be synthesized and immediately passivated with an organic coating, i.e., stabilized, so as to avoid their aggregation and to favor their affinity to the polymer. These difficulties prevent the possibility of realizing nanocomposite polymers on an industrial scale relegating the practice “ex situ” to a laboratory methodology.
The processes “in situ”, on the contrary, have proved to be much quicker and easier, since the nanometric inclusions are obtained, in the presence of the polymer, by a metallic precursor.
An example of process “in situ” is disclosed in U.S. Patent Application No. 2002/0145132 by Won et al. In such process, a composite polymer is obtained by irradiating, with ultraviolet light, a polymeric matrix containing a metallic precursor dispersed at the molecular level and reduced to metal by the radiant energy.
Although advantageous, the different processes “in situ” realized by the known technique are not exempt from drawbacks. For example, a problem shared by the different practices “in situ” is represented by the difficulty of obtaining particles of the same dimension, since the inclusions have a strong tendency towards aggregation.
A further limitation of the known processes “in situ” is the limited and forced choice of the polymers that can be used. These latter, so as to improve the compatibility between precursor and matrix, must contain hydrophilic groups which allow for an acceptable molecular dispersion of the precursor inside the polymeric matrix.
The use of such polymers is obviously disadvantageous, especially in consideration of some specific applications, since the presence of the hydrophilic groups gives the polymer a great capacity of absorbing water also when being used, with subsequent poor environmental stability of the polymeric product.
In the cases when it is possible to use non-polar polymers, as in the process “in situ” carried out by thermolysis, described in S. P. Gubin “Metal-containing nano-particles within polymeric matrices: preparation, structure, and properties” Colloids and surfaces A: Physicochem. Eng. Aspects 202 (2202) 155-163, there is however the difficulty of having to add the precursor in solution drop by drop to a polymer solution subjected to agitation.
Or still, as disclosed in the above document by Won et al., the precursor has to be dispersed or dissolved in a polymer solution. The solution then has to be cast on a support, for example a glass plate, so that it is distributed to form a liquid film allowing the solvent evaporation. The solid film obtained is then irradiated with ultraviolet light for decomposing the metallic precursors.
In practice, it is observed that such laborious techniques, due to the aggregation phenomena which can occur, are advantageous only for low inclusion concentrations; moreover, the need of working with polymeric solutions make them inadequate for use in a large scale, consequently preventing the production and the diffusion of manufactures in nanocomposite polymeric material which are, on the other hand, very advantageous.
There remains a need in the art for the preparation of a composite polymeric material having such characteristics as to overcome the drawbacks cited with reference to the processes provided by the prior art.